The symptoms that often occur with out-of-print and old cars are “the engine runs well when the RPMs are high” and “the engine tends to stall at idle”, which is a problem that I have to be careful about. Unlike injection cars, which are compensated for by the computer, carburetor cars are very delicate, so have you given up on them? In fact, it may be caused by dirt inside the carburetor. It may take some time and effort, but it can be improved by disassembling the carburetor completely and soaking it in a strong cleaner.
- Problems with carburetors that are dirty from blowback, carbon, and varnish buildup
- SSmaller jet and aisle size slow, pilot system clogs first.
Problems with carburetors that are dirty from blowback, carbon, and varnish buildup
A blow-by hose is connected to the factory air cleaner box from the crankcase for environmental protection. As combustion gases and oil mist that flow into the crankcase from the gap between the piston and cylinder during combustion and expansion of the air mixture pass through the blowby hose and air cleaner box and flow to the intake side of the carburetor, dirt, and other contaminants can adhere to the air cleaner box if the driving distance becomes long or the piston clearance becomes large. Dirt may adhere to the air intake side of the carburetor.
The characteristic of an overdrive motorcycle that is not immobile is that there is a large amount of soot dirt on the funnel side of the carburetor, but none in the float chamber.
Since the jets are not clogged and there are no problems with idling or low speeds, it is difficult to notice the dirt in the image above.
The negative pressure created by the intake air flowing through the venturi sucks up the fuel in the float chamber, and the mixture flows into the combustion chamber for combustion. This is the outline of the intake process of a carburetor motorcycle. Looking at this alone, it would seem that fuel or the air mixture is always flowing from the carburetor to the engine side, but in reality, there is also a reverse flow from the engine to the carburetor.
In a 4-stroke engine, intake air flows into the cylinder due to the negative pressure generated when the intake valve opens and the piston descends in the cylinder. Because of the inertia of the flowing intake air, the intake valve remains open even after the piston passes the cylinder’s bottom dead center, and finally closes after the piston moves into the upward travel. At this point, the force of the intake air collides with the compression of the rising piston, pushing some of the air back toward the carburetor. After combustion occurs at the upper dead center of compression, the exhaust valve opens, and a short time after the exhaust process begins, the intake valve opens to push out the exhaust gas remaining in the combustion chamber, which is also partially pushed back to the intake side by the remaining pressure in the combustion chamber. Because of this movement of air inside the intake manifold, carbon and dirt adhere to the engine side of the carburetor throttle valve.
Even if the motorcycle is not operating, the varnish that forms as the fuel in the float chamber deteriorates due to long-term storage or neglect can also cause contamination of the carburetor interior. The internal dimensions of the passages inside the carburetor are strictly controlled in order to meter air and fuel. This is why there are pilot jets, main jets, needle jets, and jet needles for the most important fuel metering. Naturally, the condition of the internal passages of the carburetor body is also important. If the fuel metered by the jets or the inside of the air jets, which are the air passages, are dirty or clogged, the amount of air and fuel passing through them will change and the correct air-fuel ratio will not be achieved, causing the engine to malfunction. Older or discontinued vehicles with accumulated dirt inside the carburetor. It is not by chance or bad luck that a carburetor that has been in storage for an extended period of time and the fuel has deteriorated will malfunction.
SSmaller jet and aisle size slow, pilot system clogs first.
The slow air jet and main air jet, which line the entrance of the carburetor, are important parts that send air to the pilot jet and main jet.
If dirt or other contaminants clog these jets, less air flows to both jets, resulting in a change in the air-fuel ratio and causing a malfunction.
The slow air jet has a smaller bore than the main air jet, so it is more affected by dirt.
When a carburetor is not working properly, most of the problems occur in the pilot and slow systems, such as “idling is not stable” and “only three cylinders burn at low throttle opening, but when the throttle opening is increased, the fourth cylinder fires” (both in the case of a four-cylinder engine). The reason for this is the difference in passage diameter between the pilot and slow systems and the main system. Comparing the diameter of the main and pilot jets, the pilot should clearly have a smaller diameter than the main.
One example of a Keihin carburetor is a combination of pilot #28 versus main #105. The numbers on the Keihin jets match the hole diameters, so the pilot would be 0.28 mm dia. and the main would be 1.05 mm dia. In diameter comparison, the difference is about 4 times larger, and in hole area, about 14 times larger. It should be easy to imagine that the pilot jet with a smaller hole diameter is more likely to clog than the main jet if left under the same conditions immersed in fuel in the float chamber.
The same is true for the air jets through which air passes. Slow air jets have smaller passages than main air jets, which are more prone to clogging. Because of these characteristics of the pilot system, irregularities are more likely to occur at idle and low throttle openings.
Of course, if the bike is ridden regularly, there is no need to worry about such irregularities.
However, if the bike has been ridden for a long time with high mileage or with settings set too high, the pilot outlet and slow port, which are the outlets for the air mixture from idling to low throttle opening, may become dirty due to blowback and carbon buildup as mentioned above, causing malfunctions. This is a problem that needs to be taken care of.